Parallel optics and wavelength division multiplexing (WDM) are two optical communication techniques that permit increased bandwidth density in optical communications systems. In parallel optics, multiple optical signals are transmitted in parallel along a multi-optical fiber ribbon, with a single optical signal being transmitted on each optical fiber. In WDM, multiple optical data signals are combined and transmitted on a single optical fiber, with each optical signal being carried on a different wavelength. In parallel WDM, the two techniques are combined by transmitting multiple optical wavelengths through each optical fiber of a parallel optical fiber ribbon. A key component of a parallel WDM system is a parallel WDM multiplexer, an optical device that combines multiple optical beams into a single optical fiber.
In some implementations of parallel WDM, the optical transmitter includes an array of vertical cavity surface emitting lasers (VCSELs). The number of VCSELs in the array is typically equal to the number of optical fibers in the optical fiber ribbon multiplied by the number of wavelengths in each optical fiber. The optical multiplexer serves to couple light from one VCSEL of each wavelength into each optical fiber in the optical fiber ribbon. Typical multiplexers use wavelength selective means such as dielectric interference filters or diffraction gratings to accomplish this.